If you have been praying for victory, [O believers] - victory has now indeed come unto you. And if you abstain [from sinning], it will be for your own good; but if you revert to it, We shall revoke [Our promise of aid] - and never will your community be of any avail to you, however great its numbers: for, behold, God is [only] with those who believe !
Al-Anfal (The Spoils of War) Verse 19
Allah's Apostle said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Apostle! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet said, "By preventing him from oppressing others."
Sahih Bukhari: Volume 3, Book 43, Number 624